Muncie 4-Speed Transmissions. Paul Cangialosi

year, is probably not a reliable source. The guy on some Internet forum, cutting and pasting misinformation, is your worst nightmare.

       This is Brian Higgins of the SK Tranny Shop. Shops like this are a great place to research castings and part numbers. Few people today will ever be able to amass this number of Muncie parts.

      Several times a day I’m asked, by email or phone, how to identify a Muncie. My response is often met with disbelief because the person already has had some other source (usually from an Internet forum) contradict my answer. Some people don’t even have a Muncie. They often get angry because they don’t want to face up to the fact that they got burned and made a bad 4-speed purchase.

      Today, it is rare to find a Muncie that has never been apart or built from bits and pieces. Untouched “survivor” Muncies are rare; so are the ones that have correct correlating casting and serial numbers.

      Main cases often suffered from broken mounting ears, but the later cases had thicker, stronger mounting ears. Its quite common to see later cases replace early ones for this reason. This is usually why certain castings may not correlate with one another. Tailhousings often get damaged where the reverse shifter shaft lock pin sits, and at both the shifter and main mount bolt-boss locations.

      Because date-coded serial numbers and VINs were only stamped (not cast) into the main case, the main case should be your starting point. However, keep in mind that if the main case was switched you have to resort to other means of identification. You can think of them as a system of checks and balances.

      Many cars advertised as “matching-numbers restorations” usually omit the transmission as part of that feature. I created a Facebook page at www.MuncieBook.com. This page is in place to support this book and it’s free. Feel free to upload pictures and ask questions. Some of the best Muncie builders participate on that page.

Muncie RPO Codes

      Just three types of Muncie 4-speeds are available based on the RPO (regular production option) code used when a car was ordered: the M20 (standard wide-ratio), M21 (close-ratio), and M22 (heavy-duty close-ratio). (The M does not stand for “Muncie.”)

      Typically, when you ordered a GM car you had standard options and accessories installed. “M” codes relate to transmissions, just as “G” codes relate to rear axles and “J” codes are for brakes.

       This 1967 Corvette has a build sheet with an RPO code for an M21 close-ratio 4-speed. Notice the other letters and the type of options they represent. Because M21 is just a code, later Corvettes or other 4-speed cars didn’t have Muncies; those equipped with a Super T10 4-speed could have an M21 or an M20 code. Window and tank sticker resources reveal that there was never an M21 option printed on a sticker before 1966, even though many M21s existed I believe the final-drive ratio determined whether a close- or wide-ratio transmission was installed.

Main Case Casting Numbers

      Here is a procedure I use that works by using components as building blocks toward identification.

      The patent that was originally filed on November 29, 1957, for the T10/Muncie design was approved 5½ years later on May 7, 1963. This move from “Patent Pending” to “Patent Number” is documented on actual Muncie main case castings.

      The evolution of case design also helps pinpoint the year a case was released. The main case casting number was used for either a specific year or series of years. Casting numbers are not GM part numbers. Some cases may look identical and have a different casting number but have the same GM part number. Usually a casting number is changed because of a design change. This could be an alloy improvement, a casting tool change, or some physical change.

      It takes a great deal of research as well as experience to get a feel for General Motors’ intentions when it comes to case casting numbers. Most of the books and articles I have seen, combined with some bits of Internet folklore, tend to have the same flaws because most information is copied and simply edited and resubmitted. I have done my own research by looking at literally thousands of cases, extensions, and sidecovers to draw my own conclusions. I can guarantee that most people reading this book will never see some of the items I’ve stumbled across.

      Muncie 4-speeds pretty much have a solid chronological order regarding the main case, retainer, midplate, and extension housing casting numbers. Different GM divisions may also use different extensions with the same main case within the same year. Casting number revisions are always chronological, which means that a higher number corresponds to a later part.

      People often get confused about what a casting number is. Simply put, it is the number that is actually cast into the case when the molten aluminum is poured into the case mold. (A serial number is stamped into the case by hand, usually with numbered and lettered punches.) As a mold is poured, date-coded clocks are cast into most cases that represent the month, week, and sometimes day the case was cast. These casting dates or “clocks” are not generally necessary to determine what type of Muncie you have. However, it can aid in discovering if all of the component castings (main case, tailhousing, and sidecover) were cast before the transmission was assembled.

Serial Numbers

      Serial numbers are stamped into the case. It is a date code for when the transmission was assembled based on the model year, not calendar year. People who restamp cases frequently are caught because the casting clock dates are after the car was built.

      Serial numbers for Muncie 4-speeds always begin with the letter “P,” which stands for Muncie Plant (not “passenger car.”) “M” and “N” were used to identify 3-speed Muncies; “O” identifies the Saginaw 3-speed with overdrive.

      Serial numbers from 1963 to 1966 included only the month and day. For example, P0101 indicates January 1. From 1967 to 1968, the serial number used year, month, and day designators; the month designator is now a letter. For example, the serial number P8A01 is January 1, 1968.